Technical Field
This disclosure relates to fasteners in general, and in particular, to aircraft skin fasteners that provide resistance to electromagnetic effect (EME) related internal damage to aircraft resulting from lightning strikes.
Related Art
In an ideal case, the EMEs of a lightning strike on an aircraft are limited to the theoretically continuous, electroconductive exterior skin thereof, in the manner of a so-called “Faraday Cage,” such that the interior of the aircraft should be relatively unaffected by the strike. However, as a practical matter, the skin, made of numerous individual panels, is actually discontinuous, and further, is pierced by a relatively large number of typically electroconductive fasteners, such as lock bolts, rivets or the like, used to attach the skin panels to associated structures, such as ribs, spars, stringers and the like. As a result, the ideal model does not hold true, and it therefore becomes desirable to develop designs and methods for protecting interior spaces of aircraft from EME-related damage resulting from lightning strikes.
FIG. 1 is lower, side perspective view illustrating a conventional “lock bolt” fastener 10, such as a HI-LOK fastener, of a known type typically used to fasten a skin panel 12 of an aircraft to an associated structure 14, such as a wing rib, spar, stringer, longeron, frame or the like, and some sequential steps involved in the attachment of the skin 12 to the associated structure 14 using the fastener 10. As illustrated in FIG. 1, the fastener 10 can comprise an elongated “pin” or shaft 16, a head 18 disposed concentrically at an upper end of the shaft 16, and lock nut or “collar” 20 having an internal thread that engages a complementary external thread disposed on a lower end portion 22 of the shaft 16. The shaft 16 and head 18 typically comprise an electroconductive metal, e.g., steel, aluminum, titanium or an alloy of the foregoing.
In a typical assembly procedure, the skin 12 and the associated structure 14 are temporarily clamped tightly together, and a common hole is bored through the two parts. In the particular example embodiment of FIG. 1, the head 18 of the fastener 10 is frustoconical in shape, i.e., intended to be countersunk below the upper surface of the skin 12, such that the upper surface of the head 18 is disposed generally flush with the upper surface of the skin 12, for streamlining purposes, and accordingly, the formation of the hole can include, or be followed by, the formation of a correspondingly shaped frustoconical counterbore in the skin 12 at the upper end of the hole. As illustrated in detail A of FIG. 1, the elongated shaft 16 of the fastener 10 is inserted into the hole until the head 18 of the fastener 10 is seated within the corresponding counterbore, and the nut or collar 20 is then started onto the lower end 22 of the shaft 16.
As illustrated in detail B of FIG. 1, a “wrenching element” 24 disposed on the outer end of the collar 20 is then engaged and rotated by an installation tool (not illustrated) to advance the collar 20 upward along the shaft 16 and toward a lower surface of the associated structure 16, as indicated by the arrow 26, while the shaft 16 is prevented from rotating by the engagement of a socket 27 in the lower end of the shaft 16 with the installation tool. The wrenching element 24 is integrally coupled to the collar 20 through a stress-raising feature 28, such as a notch or groove. As illustrated in detail C of FIG. 1, continued rotation of the wrenching element 24 and collar 20 relative to the fixed shaft 16 with the installation tool eventually drives the upper end of the collar 20 into engagement with the lower surface of the structure 14 with a preselected amount of clamping force, at which point, the collar 20 becomes swaged into a locked position on the shaft 16 and against the associated structure 14, while the wrenching feature 24 is sheared away from the collar 20 at the stress-raising feature 28 and subsequently discarded. Thus, in an installed state, a portion of the shaft 16 disposed below the lower surface of the head 18 and above the upper end of the nut or collar 20 is loaded in a preselected amount of tension, while the skin 12 and the associated structure 14 are pressed together by the fastener 10 with a corresponding amount of compressive force.
In another known type of lock-bolt fastener that does not involve rotation of the collar 20 relative to the shaft 16, such as a HUCK fastener (not illustrated), the shaft 16 can comprise a breakaway extension, or “pin-tail,” disposed on a lower end of the shaft 16 that is gripped by an installation tool and pulled axially outward relative to the collar 20, such that the collar 20 is forced axially over a series of thread-like corrugations on the circumfery of the lower end of the shaft 16 and into a compressive engagement with the lower surface of the structure 16. As above, when a pre-selected amount of compressive force on the skin 12 and associated structure 14 has been reached, the collar 20 becomes swaged onto the shaft 16 and against the associated structure 14 in a locked position, and the pin-tail on the shaft 16 is sheared away and discarded.
Another type of conventional aircraft skin fastener 10 commonly used in lighter aircraft applications is a rivet, which, like the lock-bolts described above, comprises an elongated shaft 16 and a head 18, but which omits a nut or collar 20 in favor of a radial expansion of the lower end portion 22 of the shaft 16 as a technique for compressing the skin 12 and the associated structure 14 together, which expansion can be effected through a variety of well-known mechanisms.
While the first embodiment of skin fastener of FIGS. 3A, 3B and 4 can provide good protection against lightning-induced EME-related damage to interior spaces within aircraft having conventional metal, e.g., aluminum alloy, skins, its use, like that of the conventional fastener of FIGS. 1 and 2, may be contraindicated in some aircraft with composite skins, i.e., skins made of laminated sheets of carbon fibers embedded in a relatively soft, dielectric polymer-resin matrix, because the elongated, relatively hard shafts of the fasteners can have a tendency to gall or seize when inserted into the corresponding bores formed in the skins to receive them, resulting in a deformation of the bore and an abrading-away of the relatively softer material of the composite skin into the interface between the fastener and the skin.
However, as illustrated in FIG. 5, it is known that this problem with composite-skinned aircraft can be successfully addressed by the provision of a complementary, thin-walled, metal sleeve 244 disposed concentrically around the shaft 216 and below the head 218 of each fastener 200. The sleeve 244, which can include an upper end portion 246 that is complementary in shape to the head 218, can be inserted into the corresponding bore in the aircraft skin before the body of the associated fastener 200 is inserted therein. The thin wall of the sleeve 244 enables it to deform radially inward when being inserted into the corresponding bore, rather than galling the adjacent skin. The body of the fastener 200, i.e., the shaft 216 and the head 218, can then be inserted into the relatively hard sleeve 344 without galling.
Many skin fasteners like those described above and currently used in commercial airplanes are not designed for problems caused by lightning strikes because they do not allow for the safe escape to the atmosphere of superhot EME-related gases and particles that can be generated in the interfaces between respective ones of the fasteners and the aircraft skin surrounding them. This can be critically important in locations near or within the wing tanks of aircraft, where a lightning strike could result in a particularly undesirable outcome, and this is true whether the aircraft skin comprises a metal, e.g., an aluminum alloy, or a composite material, such as a carbon-fiber/resin layup. Thus, while there are designs for accommodating lighting strikes in airplanes, alternative or additional systems are needed.
Prior art efforts have attempted to address the problem, for example, by insulating the upper surfaces of the fasteners with a layer of a dielectric material, by making all or a portion of the fasteners themselves of a dielectric material, or by encapsulating the lower end portions of the fasteners in a dielectric material. While these efforts in some cases have met with some measure of success, none provides for the venting to the atmosphere of pressurized, superhot EME-related gases and particles resulting from a lightning strike, while simultaneously blocking the flow of those gases and particles to interior spaces of the aircraft.
Accordingly, a long-felt but as yet unsatisfied need exists in the industry for aircraft skin fasteners that are strong, reliable and cost effective as conventional fasteners, yet which also provide robust protection against EME damage to interior spaces of an aircraft, including metal- and composite-skinned aircraft, resulting from lightning strikes.